The human knee-joint comprises two pairs of tibio-femoral articular surfaces, namely a medial pair of articular surfaces and a lateral pair of articular surfaces, which are in each case formed by a femoral condyle and, respectively, by a dish-shaped tibial or meniscal articular surface interacting with said femoral condyle. In prosthetic simulation of this joint, the femoral slide surfaces are formed either by a one-piece prosthesis part or by two separate prosthesis parts. The latter are referred to as sliding prostheses. To ensure that the femoral and tibial articular slide surfaces can interact correctly, their lateral spacing must be identical. When using sliding prostheses, this means that their spacing must be exactly maintained. For this purpose, insertion instruments are known (EP-A-1 099 430) in which two clamps are arranged on a supporting plate of the instrument and are each intended to receive a sliding prosthesis. The spacing between these clamps can be adjusted according to the requirements of the associated tibial prosthesis part. The clamping jaws of the clamps each interact with the side edges of the sliding prostheses. Since force transmission is limited at this position, a vertically adjustable slide-surface support is also provided in each clamp and takes up a large part of the force transmitted from the instrument to the prosthesis parts during implantation.
In the known instrument, one clamping jaw of each of the clamps is connected to the supporting plate in a nonadjustable manner and it supports the laterally adjustable second clamping jaw. If one wants to change the spacing between the clamps, another supporting plate or other clamps are chosen. A supply of different supporting plates or clamps must therefore be kept. This is expensive. It is also difficult, during the operation, to choose a clamp spacing different than the one selected during the preparations for the operation. The arrangement of the slide-surface supports on the clamps is disadvantageous because their arrangement is complicated and access to them difficult.
In another known insertion instrument for sliding prostheses (U.S. Pat. No. 3,949,428), the clamps themselves are not adjustable. Instead, the clamping jaws have guide tracks which interact with corresponding grooves in the sliding prostheses. To ensure that they can be released again from the sliding prostheses after the latter have been implanted, the grooves on the sliding prostheses must be open at the end. Since they cannot open into the slide surfaces themselves, this solution cannot be employed in the sliding prostheses customary today. The spacing of the clamps from one another can be adjusted because they are arranged displaceably on a holding frame and each have a locking screw by means of which their respective chosen position can be fixed. This is a very elaborate arrangement which scarcely permits the spacing to be changed under the conditions existing during surgery.